CHAPTER V

THE DEVELOPMENT OF ANTS

"Incredibili Topy et cura Formicæ educant summamque dant operam, ne vel tantillum quod spectet eorum Vermiculorum educationem atque nutritionem omittant."-Swammerdam, "Biblia Naturæ," 1737.

"Ces insectes, si peu timides et qui ne craignent point pour eux-mêmes les intempéries de l'air, sont d'une extrême sollicitude pour leur petits, ils redoutent pour ces êtres, d'une constitution délicate, les plus légères variations de l'atmosphère; s'alarment au moindre danger qui semble les menacer, et paroissent jaloux de les soustraire à nos regards."-P. Huber, "Recherches sur Les Mœurs des Fourmis Indigènes," 1810.

Ants, like other metabolic, or metamorphosing insects, pass through four consecutive stages, or instars, before reaching their adult, or imaginal form. These stages, which are known as the egg, or embryo, the larva, the semipupa, or pseudonymph and the pupa, or nymph, are very similar to those of other Hymenoptera both social and solitary. Such close adherence to an ancient method of development in insects, which, in their adult stage, present so many idiosyncrasies of structure and behavior, must be attributed to a general principle according to which the developmental stages of an organism are much more conservative than the adult. The highly modified behavior of the ants themselves towards their brood certainly contrasts very forcibly with the monotonous repetition in the young of stages essentially like those of solitary wasps and gall-flies. This contrast becomes more intelligible, however, when we follow the various phylogenetic stages through which it has been attained. Even the solitary insects make some provision for their young by placing their eggs in suitable situations, and while, among insects of the lower orders, these situations represent merely an indefinite environment, like the earth or the water, in which the young will have to seek their food, the lower Hymenoptera (saw-flies and gall-flies) deposit their eggs only on certain plants. The solitary wasps and bees make ampler provision in constructing. cells for the individual larvæ and in supplying them directly with prepared foods. In all of these cases the relation of parent to offspring is both protective and nutritive, but the protective relation is still incompletely developed, since these insects are unable to remove their brood when the nest is disturbed or destroyed. The ants, however, have entered into much more intimate relations with their progeny. They

never construct elaborate earthen, paper or waxen cells for the individual larvæ, and, unlike the solitary bees and wasps, which never see their brood, or the social bees and wasps, whose experience is

FIG. 34. Interior of a formicary to show the classification of the larvæ and pupæ according to their stages. (Ern. André.)

largely confined to the heads and gaping mouths of their progeny, the ants have acquired an extensive and uniform experience with all the developmental stages of their species from the egg to the adult.

They not only feed, but clean and transport the young from place to place, thus utilizing to the advantage of their development the evervarying temperature and humidity of the soil. By this means they also protect them from exposure to light and enemies. Moreover, they assist the young to undergo their transformation by embedding them in the earth till the cocoons are woven, and eventually extricate the hatching callows from their envelopes. This freedom in dealing with the brood is certainly one of the most striking manifestations of the plasticity of ants. The remarkable consequences which it entails in their relations with other insects will be

considered in future chapters.

As the eggs, larvæ and pupa develop in the dark recesses of the nest, these stages are always of a pale color, usually translucent white or yellowish, more rarely greenish or roseate, like the corresponding stages of other insects that develop in dark cavities of the soil or in the tissues of plants or animals. Ants rarely or never bring the brood to the surface unless they feel compelled to move to another nest or belong to species like the slave-makers, which kidnap the young of other ants. Occasionally, however, during very warm weather, the young may be brought to the surface after nightfall. In the dry deserts of western Texas, I have seen Ischnomyrmex cockerelli bring its larvæ and pupæ out onto the large crater of the nest about 9 P. M. and carry them leisurely to and fro, much as human nurses wheel their charges about the city parks in the cool of the evening.

Since the brood is always nurtured in darkness we must suppose that the manipulation which this implies depends on highly developed tactile and olfactory senses to the exclusion of vision. Evidence of the exquisite perfection of these senses of contact-odor is seen in the segregation of the brood according to age and condition. The eggs, larvæ and pupe of different sizes are placed in separate piles in the same or different chambers of the nest, reminding one, as Lubbock (1894, p. 7) aptly says, "of a school divided into five or six classes" (Fig. 34). Inspection of the nests of many species of ants shows that this habit is very prevalent, although it is not so clearly manifested in primitive.

groups (Ponerinæ) or in species that form small colonies, as in the opulent formicaries of the more highly specialized genera (Myrmica, Aphanogaster, Formica, Camponotus, etc.). This classification seems to be an expression of a need for different degrees of moisture and temperature in different developmental stages, as Janet has shown (1904, pp. 38, 39). He says: "In regard to the degree of humidity. most favorable for each class of progeny, I have made the following observation on artificial nests of a porous substance, in which the humidity was very regularly graduated, and containing a populous colony of Myrmica rubra lævinodis, with extremely numerous offspring. The

larvæ of medium and large size had been placed on the floor of a very damp chamber. In the less humid neighboring chamber, enormous packets of eggs were found at the bottom of the wall, and above them, attached by their hooked hairs, were all the just-hatched larvæ. All the pupa were in the even dryer adjacent chamber." As the ants are continually shifting their young about in the nest in response to diurnal changes of moisture and temperature, bringing them nearer the surface during the warm hours of the day and carrying them below during the cooler nights, the classification in wild colonies is best seen only when the weather has been unusually constant for several days.

The eggs of ants are minute bodies, hardly more than .5 mm. long even in the largest species, and usually much smaller. They are commonly overlooked by the casual observer who applies the term

eggs" erroneously to the cocoons or even to the larvæ and pupæ of many of our species. In a few groups, like the Attii, the eggs are nearly spherical or broadly elliptical, but in most species they are elongate elliptical (Fig. 45, a). In certain Ponerinæ (Parasyscia, Lobopelta, Ponera, etc.) they are unusually long and slender and may be described as cylindrical (Figs. 37, a, and 40, a). The yolk, like that of the bee's and wasp's egg, is very thin and liquid and is enveloped by a delicate, transparent shell, or chorion. As in other elongated insect eggs, the longitudinal axis of the future embryo and adult insect is clearly predetermined and corresponds with the long axis of the egg. One of its poles therefore foreshadows the anterior, or cephalic, the other the posterior, or caudal end of the ant. There are said to be no differences in the eggs corresponding to the castes into which they develop, but this matter requires further investigation. It is certain that the eggs deposited by the same female often vary considerably in size and shape, and those laid by the workers are sometimes only half as large as those laid by females of the same species. As the ants frequently lick the eggs it is possible that the saliva may be absorbed by osmosis and increase their volume. This salivary coating is also important in causing the eggs to cohere in packets so that they can be quickly and easily carried away in case of danger. It is probable, moreover, that the saliva contains some antiseptic substance which prevents the destruction of the eggs by fungi.

FIG. 37. Parasysci augusta. (Original.) a, Eggs; b, young larva, lateral view.

While the eggs are passing out of the oviducts of the female they may be fertilized with some of the spermatozoa stored in the spermatheca, or they may pass the orifice of this organ and escape from the body without fertilization. The latter, is, of course, always the case in old females whose supply of spermatozoa has been exhausted, or in workers, which usually lack the spermatheca and are not known to mate with males. According to a well-known theory, advanced by Dzierzon for the honey-bee, the unfertilized eggs develop into males, the fertilized eggs into females or workers. Although it has beer. shown by a number of authors, and especially by Miss Fielde (1905f), that unfertilized eggs develop into males, Tanner (1892), Reichenbach (1902), and Mrs. Comstock (see Wheeler, 1903a, pp. 835, 836) have recorded observations which indicate that the unfertilized eggs of